to reinstall macOS big sure on MacBook Air : utilities window choose Utilities > Terminal from the menu bar Type resetpassword in Terminal Click on the Reset Password box(any where)  and from menu bar select Recovery Assistant > Erase Mac from the menu bar (even if there no user inside the box) 4. Click Erase Mac in the window that opens When done, your Mac will restarts. 5. follow the steps in wizard , Until you see Exit to Recovery Utilities. 6. select Reinstall Mac OS Big Sure